EVANSTON, Ill. - In a paper published Dec. 14 in the journal Nature, Northwestern University geologist Craig R. Bina reports that, in a novel twist on current thinking, water may be transported into the interior of planets as a high-pressure form of ice, rather than simply being transported while trapped within hydrous minerals or escaping as a fluid.

Bina and co-author Alexandra Navrotsky, a chemist and materials scientist from the University of California, Davis, suggest that this process should become more important as planets cool, for example on a future Earth or on Mars.

Large amounts of a form of water ice, called "ice VII," may be accumulating deep in the Earth's interior during the process of subduction, where the oceanic crust is stuffed periodically below the surface crust. (Subduction is one of the main types of action in plate tectonics.) The most likely place to look for evidence of such a process, say the researchers, would be in Tonga, a group of islands in the southwest Pacific Ocean and one of Earth's coldest subduction zones.

The researchers' calculations indicate that in these subduction zones conditions do exist that could result in the formation of ice VII, a form of ice in which the atoms are packed closely together. The high pressures and low temperatures beneath the surface in an area like Tonga could result in water being carried deep below the Earth's crust as ice - which is not very mobile - before temperatures are great enough to melt the ice and enable it to be released as water.

"To my knowledge, we are the first to look at the possibility of the water being released from the subsumed rock as ice, not as a fluid," said Bina. "Tonga would be the most obvious place to look for geophysical evidence of ice VII."

The decreasing availability of fluid water, caused by the accumulation of ice VII and its subsequent reaction products in a cooling planetary interior, as in the future Earth or Mars, might eventually lead to a decline in tectonic activity or its complete cessation.

"Recent discovery of what look like gullies on Mars suggest that the planet is not completely dry," said Bina. "Water for forming gullies could be stored at shallow levels beneath the Martian surface, perhaps as normal ice. Our work suggests that there could also be water stored in the deep interior as high-pressure ice, which could be released to drive volcanic activity."

Bina and Navrotsky suggest that the state - fluid versus crystalline - of water inside planets can profoundly affect both dynamics and thermal balance, and that a change in the relative proportions of fluid and solid water "is one of the important factors influencing the different possible paths of planetary evolution."